1. Field of the Invention
The present invention relates to optical communications systems. More particularly, the invention relates to optical amplifiers and to modules using them.
2. Related Arts
Optical communications systems are rapidly becoming a widespread and important technology in telecommunications and networking. Optical communications systems transmit information optically at very high-speeds over fiber optics. The key optical components of an optical communications system include optical amplifiers, in particular, semiconductor optical amplifiers. In optical communications systems, optical amplifiers are used to, for example, attenuate optical signals therein.
There have been known semiconductor optical amplifiers. One representative type of semiconductor optical amplifier comprises an optical cavity which resembles that of a semiconductor laser and is operated below the lasing threshold. Another representative semiconductor optical amplifier is a tunable-gain semiconductor optical amplifier which controls the gain in the active region. In the former example, carriers are pumped by injecting current into the optical cavity. As the optical signal passes through this region, it is amplified based on the emission stimulated by pumped carriers. One example of the latter type comprises an optical cavity which lases in the substrate's vertical direction perpendicular to the optical axis of the optical signal. The gain in this active region is controlled. Another gain-tunable semiconductor optical amplifier is also known which is a gain-fixed semiconductor amplifier connected in series with a variable attenuator. These examples are disclosed in such documents as U.S. Pat. No. 6,347,104 entitled “Optical signal power monitor and regulator” (Patent Document 1) and U.S. Pat. No. 6,445,495 entitled “Tunable-gain semiconductor optical amplifier” (Patent Document 2).
[Patent Document 1]
U.S. Pat. No. 6,347,104 (lines 14 to 43 column 16, FIGS. 3A and 3B)
[Patent Document 2]
U.S. Pat. No. 6,445,495 (lines 24 to 46 column 2, FIG. 8A)
One problem with conventional optical amplifiers is that the gain changes depending on the intensity of the incident optical signal and is not saturate. Although gain-tunable semiconductor optical amplifiers have been proposed to cope with this problem, these examples have yet to solve such problems as spectrum broadening due to spontaneous emission and rising of the noise level caused by the broadening spectrum. In the case of a semiconductor optical amplifier connected in series with a variable attenuator, it is involved with yet another problem of increased elements.